Natural gas generally refers to rarefied or gaseous hydrocarbons (comprised of methane and light hydrocarbons such as ethane, propane, butane, and the like) which are found in the earth. Non-combustible gases occurring in the earth, such as carbon dioxide, helium and nitrogen are generally referred to by their proper chemical names. Often, however, non-combustible gases are found in combination with combustible gases and the mixture is referred to generally as “natural gas” without any attempt to distinguish between combustible and non-combustible gases. See Pruitt, “Mineral Terms-Some Problems in Their Use and Definition,” Rocky Mt. Min. L. Rev. 1, 16 (1966).
Natural gas is often plentiful in remote locations or regions where it is uneconomical to develop those reserves due to lack of a local market for the gas or the high cost of processing and transporting the gas to distant markets. Such natural gas is accordingly referred to in the energy industry as “stranded gas” or “remote gas”.
Recently a number of methods have been investigated and/or proposed to allow for more economic use of such resources by converting the stranded gas into products which are more readily transportable, such as methanol, dimethyl ether or other chemicals, as well as liquid hydrocarbons.
It is also commercially important to cryogenically liquefy natural gas so as to produce liquefied natural gas (“LNG”) for more convenient storage and transport. A fundamental reason for the liquefaction of natural gas is that liquefaction results in a volume reduction of about 1/600, thereby making it possible to store and transport the liquefied gas in containers at low or even atmospheric pressure. Liquefaction of natural gas is of even greater importance in enabling the transport of gas from a supply source to market where the source and market are separated by great distances.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to extremely low cryogenic temperatures of from −240° F. (−151° C.) to −260° F. (−162° C.) where it may exist as a liquid at near atmospheric vapor pressure. Various methods and/or systems exist in the prior art for liquefying natural gas or the like whereby the gas is liquefied by sequentially passing the gas through a plurality of cooling stages, and cooling the gas to successively lower temperatures until liquefaction is achieved. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, nitrogen and methane, or mixtures thereof. The refrigerants are commonly arranged in a cascaded manner, in order of diminishing refrigerant boiling point. As appreciated by those skilled in the art, LNG plants are relatively expensive to build and operate. Further, the resulting LNG product must be transported in specially designed ships to maintain the LNG in liquid form for extended periods of time at such cryogenic temperatures until it reaches a market, where it then must be regasified in a specialized regasification facility.
Dimethyl ether can be manufactured from natural gas, coal and other carbonaceous feedstocks and is used in some markets as a fuel or fuel blendstock. See e.g., U.S. Pat. Nos. 4,341,069; 4,417,000; 5,218,003; and 6,270,541, and European Patent Publications 0 324 475 and 0 409 086, the teachings of which are incorporated herein by reference.
Many current or potential markets for dimethyl ether that is used as a fuel, such as in China, India, Japan, Europe, and Korea, are located significant distances from natural gas resources that could supply the demand for such fuel, such as inland natural gas fields in central Russia.
To economically supply markets for dimethyl ether where it is to be used as a multi-purpose fuel, the feedstock resource such as natural gas or coal generally needs to be located at or close to a coastal location so that the dimethyl ether produced with such feedstock can be economically transported to distant markets by ship. If the feedstock resource is located at for example a remote inland area; that is a significant distance from the coastal location, then transport options for dimethyl ether produced at such remote locations, such as by a dedicated pipeline, railroad car, or trucks, may make it uneconomical to transport the dimethyl ether into a relevant fuels market. In addition, if the dimethyl ether is manufactured within or close to the relevant fuels market location, and the natural gas available for use as a feed for making dimethyl ether in that location has been transported there as LNG or by pipeline, the natural gas within that market location may also be too expensive to economically convert the gas to dimethyl ether in that market location for use as a fuel since a significant amount, such as about 30%, of the natural gas is used for process fuel; that is, only 70% of the gas is utilized to make dimethyl ether. In many cases it would be more economical to produce the dimethyl ether at or close to the area where the natural gas is produced. However, as mentioned, transport of the resulting dimethyl ether to a distant market in those cases is a practical problem.
In addition to dimethyl ether, the manufacture of other compounds, such as olefins, paraffins and aromatic hydrocarbons, by the known methanol-to-olefins (“MTO”), methanol-to-gasoline (“MTG”), or by Fischer-Tropsch (“FT”) type processes are increasingly becoming important, particularly with respect to conversion of remote carbonaceous feedstocks, such as biomass, coal and natural gas, to liquid fuels. Such processes are described for example in U.S. Pat. Nos. 3,928,483; 5,177,114 and 6,743,829, the teachings of which are incorporated herein by reference. The same transportation concerns mentioned previously for the manufacture of dimethyl ether from remotely located carbonaceous feedstocks would also apply to the manufacture and use of these other synthetic products for fuels applications.
U.S. Pat. No. 6,632,971 discloses a process for converting natural gas to methanol in liquid form at a remote natural gas production site and transporting the methanol by truck, tanker, supertanker, and pipeline to a refinery where the methanol is converted to fuel products or petrochemicals. Such transportation of liquid methanol has its drawbacks, such as described previously for dimethyl ether, and particularly with respect to use of natural gas resources remotely located in inland areas. Further, methanol can be corrosive and more difficult to handle. The patentees of the '971 patent also state in a comparative example that production of ethylene and propylene at the production site for the natural gas feed is not desired as those products cannot be shipped economically.
U.S. Pat. No. 6,449,961 discloses a method for transportation of light hydrocarbons by compressing them into a so-called “dense phase” state which is said to enable the hydrocarbons to be shipped via a transport vessel, i.e., a ship. The relevant teachings of U.S. Pat. No. 6,449,961 are incorporated herein by reference. While such method is said to reduce the size of cooling systems associated with current transportation technologies, the method relies upon transportation vessels, such as ships, rail cars or trucks, which are not always reliable and are still subject to weather concerns.
In some cases natural gas, which may include natural gas liquids or NGLs therein, produced from a subterranean reservoir, is sent via pipeline in a dense phase state in order to increase pipeline capacity. One such system is the Central Area Transmission System (CATS) wherein natural gas produced from various locations in and around the North Sea is collected and conveyed by pipeline in a dense phase state to natural gas processing facilities in the UK. Another example is the Alliance natural gas pipeline system located in Canada.
As can be seen, it would be desirable to develop alternatives for transport of such synthetic products, such as dimethyl ether and hydrocarbons produced by a MTO, MTG, or FT hydrocarbon synthesis. Such alternatives could make such remote carbonaceous feedstocks, such as coal, biomass, or natural gas, and the resulting synthetic products produced therefrom, a more economical and commercially attractive energy resource from the perspective of both energy producers and consumers.